1. Field of the Invention
The present invention relates to means and a method for accurately measuring pressure with a hot cathode ionization gauge.
2. Discussion of Prior Art
Prior art hot cathode ionization gauges are calibrated by measuring the current i.sub.+cal to the ion collector electrode, at a fixed known value of electron emission current i.sub.+cal, at known pressures P.sub.cal, in the calibration system. A gauge sensitivity S is then defined where ##EQU1##
In order to measure an unknown pressure P.sub.X in a vacuum system, the current i.sub.+X to the ion collector electrode is measured using an emission current value of i.sub.-cal. It is then assumed that the unknown pressure P.sub.X can be calculated using Eq. 2. ##EQU2##
It has long been recognized that Eq. 2 does not give accurate results but surprisingly little has been done to improve the accuracy of measurement.
The root cause of the problem with accuracy is that because of historical precedent, ionization gauges are calibrated in units of pressure whereas all ionization gauges measure gas density. Under conditions of thermal equilibrium, pressure P and gas density n are simply related by Eq. 3. EQU P=nkT (3)
where k is the Boltzmann constant, and T is the absolute temperature of the gas. However, pressure and density are not simply related variables in a hot cathode ionization gauge because conditions for thermal equilibrium are not present in an operating hot cathode ionization gauge and an absolute temperature cannot be defined.
It is instructive to examine in some detail why Eq. 2 does not give accurate results.
If Eq. 1 is substituted into Eq. 2, the result is ##EQU3##
The conventional way of interpreting this result is that when EQU i.sub.+X =i.sub.+cal (5)
then Eq. 6 must hold. EQU P.sub.X =P.sub.cal (6)
This interpretation is the basis for known prior art hot cathode ionization gauge calibrations. However, Eq. 4 also implies that if Eq. 6 holds then Eq. 5 must hold. Because the ion collector current in a hot cathode gauge is a function of the gas density n, from Eq. 5 we must have EQU n.sub.X =n.sub.cal (7)
Here, n.sub.N is the number of gas molecules per unit volume in the gauge at the unknown pressure P.sub.X and n.sub.cal is the number per unit volume present in the gauge when the calibration pressure was P.sub.cal.
If the interior surfaces of the gauge exposed to the ion collection volume are not at substantially the same temperature during measurement of P.sub.X as during calibration at P.sub.cal, then the gas molecules incident on the surfaces will have different average kinetic energy when they leave the surfaces, therefore, different average velocity during measurement than was present during calibration. Therefore, the transit time for gas molecules through the ion collection volume will not be the same during measurement of P.sub.X as during calibration. If the transit times are not the same, then the number of molecules per unit volume which are present will not be the same and Eq. 7 is not satisfied.
For Eq. 7 to be satisfied requires that the average energy of the gas molecules in the ion collection volume during measurement of an unknown pressure P.sub.X be substantially the same as that prevailing during calibration at substantially the same pressure. If the average energy is different, then Eq. 7 is not satisfied.
There is considerable prior art on how to correct pressure measuring transducers for the effects of temperature changes. See, for example, U.S. Pat. No. 4,468,968 wherein it is taught how to correct for the effects of temperature change on the transducer elements per se but not on changes in the medium being measured. In a hot cathode ionization gauge ambient temperature changes have negligible effect on the performance of the gauge itself but can have substantial effects on the gaseous medium being measured and, therefore, on the output of the ionization gauge.
In U.S. Pat. No. 4,866,640, Morrison teaches that the effect of a different gas temperature during use than was present during calibration of a hot cathode ionization gauge can be corrected for from gas temperature measurements. The ratio of the absolute gas temperature, T.sub.cal, measured during calibration divided by the absolute gas temperature, T.sub.use, measured during use is multiplied by the value of gauge sensitivity, S.sub.cal, obtained during calibration to obtain a corrected value of gauge sensitivity, S.sub.use. See Eq. 16 in Morrison.
A fundamental error in the teachings of Morrison in U.S. Pat. No. 4,866,640 is the assumption that an absolute gas temperature can be defined in a hot cathode ionization gauge. The gas temperature as used by Morrison can only be defined for conditions of thermal equilibrium. However, thermal equilibrium is not present in a hot cathode gauge where there is net heat flow between numerous parts.
Another error is the assumption that a gas temperature can be measured practically in a hot cathode gauge. Although Morrison specifies a gas temperature measuring element, there is no teaching of how the gas temperature can be measured at the low pressures of interest where the mass of all the gas in the gauge is orders of magnitude below that of any known temperature sensor. For example, at 1.times.10.sup.-10 Torr the total mass of gas in a gauge is only of the order of 10.sup.-14 gram. An equal mass of tungsten would have a volume of approximately 10.sup.-15 cm.sup.3. The heat content of all of the gas in the gauge at low pressure is orders of magnitude less than that in the smallest gauge part and will have no effect on a practical gas temperature sensor.
Morrison also ignores the presence of radiated energy from the incandescent cathode. All surfaces within a hot cathode gauge are bathed in radiant energy from the hot cathode which will affect the temperature of any gas temperature sensor many orders of magnitude greater than will the relatively few gas molecules present at low pressure. For this further reason, gas temperature cannot be measured in a hot cathode ionization gauge in a practical way.
Another error is that Eq. 3 applies in a hot cathode ionization gauge. Equation 3 above holds only under conditions of thermal equilibrium and thus Morrison is in error in using this simple relationship between pressure and temperature in a hot cathode ionization gauge where thermal equilibrium does not exist.
In U.S. Pat. No. 5,250,906 where one of the inventors is the present applicant, claim 8 thereof recites, inter alia, that a reference gauge used during calibration has the same sensitivity at any given pressure and cathode heating power as a predetermined gauge used to measure unknown pressure.
However, applicant has found numerous instances in which the reference gauge had substantially the same sensitivity at any given pressure and cathode heating power as did the predetermined gauge at one time and not at other times. Thus, U.S. Pat. No. 5,250,906 does not teach how to cause the predetermined gauge to behave consistently like the reference gauge or vice versa.
The concept of gauge sensitivity S has been universally used in prior art gauge calibration methods with and without corrections to S for changes in surface temperatures within the gauge. Applicant has discovered that S is a complicated function of the temperature and area of the surfaces exposed to the gas in an ionization gauge and, therefore, is not an appropriate parameter for use in accurate pressure measurement.
Furthermore, applicant has found a new method of calibrating hot cathode ionization gauges which completely avoids the use of the concept of gauge sensitivity and any need to measure gas temperature, therefore, avoids the complications introduced into pressure measurement when S changes.